SMT Diode id & home automation system repair

Hey, that's not bad at all! Congrats!
Well, at least you'll have the parts if it decides to blow a few years down the line.
Do put something in series with the wiring to the sounder, just to be safe.
It could be interesting to see what the upgrade might consist of. Maybe there was a weak spot.

You had recommended a test light and that is a good suggestion, but I think that I will brave crawling through the rafters, yet again, to rewire with a larger gauge wire. Apparently 24 gauge is too thin for this application - 20ga or 18 would be a better choice according to manufacturer, although it did work! Albeit briefly. :O As for the upgrade, I havent received it yet, but judging from a picture, all the parts look the same. It would be interesting to see if the diode that blew along the burnt trace is the same or an upgrade. Will let you know as soon as I get it

I received the board - the trace was jumpered via a thin wire and the LM was replaced. Everything else on the board is the same except for an upgraded IC. I installed the unit and left out the sounder to perform a power up check. I think there is still an issue. There is a small disc type capacitor that gets hot to the touch very quickly! It is located very close to the LM that was repaired. I will get a photo up of it in the next few days... Needless to say, I powered down and started to troubleshoot a little. I removed everything and powered the system up bare (no inputs or outputs just the board - disc cap was ok, didnt heat up). I tried with just the sensors attached, same, disc cap. ok. When I added the two keypads to the load, the cap started to get very hot... The wiring to the keypads is also cat5 and according to spec should be larger gauge. I think that my first step is to rewire those and then see if I still have the same problem. Thats where I am at in the saga... any thoughts?

Thanks for keeping up with me

No prob's!
Hmm, I see no disc cap, only F1 & F3. And I can't see thin wiring causing an overload.

It is F3 that is getting hot. I am off of work tomorrow, so I will power up the board with one keypad and see if it gets hot. If it doesn't I will check the draw on the keypad to see if maybe the cumulative draw between two keypads and all the sensors is causing the cap to get hot... The system worked for two + years before so I am not sure that this is the issue, however, it might have been hot before and I never noticed it either....
If the 1 keypad causes the cap to get hot, I can remove the keypad from the wall and wire it at the board location with a real short run and see if that makes a difference. I will let you know tomorrow
- John

The F's are PTC fuses that are supposed to get hot as they approach their spec'd current.
But I came to think of one possible reason for thin wiring overload, and that is if the load has a built-in switching regulator.
Say the keyboard is supposed to receive 12V but runs on 5V 1A internally. It would try to draw 5.5-6W from the supply regardless of its voltage.
So if it gets 12V it would draw 0.5A but if it receives only 6V it would draw 1A. The main board would have to supply 1A at the full 12V's - making 12W.

Interesting point, I will check what the voltage going out to the keypads is registering at!

The plot thickens....
So, the F3 cap gets warm only with one specific keypad and not the other. I removed the affected keypad from wall, checked continuity on wiring (looped 2 pairs of wire, each had about 3.4 ohms resistance per leg) - ok. Backside of keypad shows a burnt diode! So that is why I am getting the F3 hot. It must have burnt out at the same time the mainboard blew out. The question is now, how they worked together to kind of see why that occured and to prevent it from happening again. This was not the keypad that I was using when the board blew so it wasn't like it was actively being used to draw more current. It is about 30ft longer of a wiring run than the other keypad. The diode that blew on the keypad is a smc made by General Semiconductor with markings of BM then directly underneath that 45 and the General Semi logo. I have asked DigiKey to id the component for me because I couldn't find any more info on it as far as what would replace it.
Here is a photo of the keypad


A close up of the damaged area, I couldn't get it clearer but its D5 that is burn out.



One side of D5 has continuity with the red lead. The black lead has continuity with D4,3,5 on the side closest to the edge of the board.

Always nice to find the reason behind problems.
I find that it's practically the same transient protection diode as on the main board.
SMBJ15A = SMBJ15CA = SMB = LM = BM
Being connected across the power (?) confirms this function.
You say you looped 2 pairs of wire for measuring, each having about 3.4 ohms resistance per leg. Does that mean the panel in normal use sees 3.4 ohms worth of wiring for its power supply? This would give a short-circuit current of 13.8V/3.4=4A.
But if the diode is shorted how could the panel work? (Or did it?)
I don't see any switchmode regulator on the panel btw., only a linear one if that's what it is.

Well, at least getting closer to finding the problem

I am with you on converting the part number from the smbj15 until you get to the last step. I see it referenced in the datasheets to LM, but how did you make the last leap to BM. Also I couldn't see a marking to tell me where the cathode is, does that mean that the part is actually bi-directional?

Yes, D5 has continuity with the red lead on the interior side of the board while the black lead has continuity with the side closest to the edge of the board.

There where 2 pairs of wires coming in, so I looped them to create continuity and tested back at the main panel. There was continuity and the total reading per pair was 3.4 ohms or 1.7 ohms per leg. (i.e. red lead would have 1.7ohms resistance and so would the black lead)

The diode is toast, but the odd thing was that the keypad still lit up and display looked as if it was receiving data from the main board! There was an alarm from the keypad when the unit was powered on whereas the other keypad did not set off an alarm, I assume to warn the user of a fault.

Yes, below C4 is a LM78MOS cdt. What are your thoughts on why you are asking about whether there was a switchmode regulator on the keypad or not? Were you thinking if there was one then the full power would not have been drawn onto the board?

Thanks again